As a scheme to reduce the cost for configuring a radio communication system, a distributed antenna system (DAS) has been implemented. In the distributed antenna system, a signal processing device that processes a transmission signal and a radio device that outputs a radio signal are separated. In the following description, the signal processing device may be referred to as a “digital processing unit”. The radio device may be referred to as a “remote radio unit (RRU)” or a “remote radio head (RRH)”.
A transmission between a digital processing unit and a remote radio unit is implemented by, for example, radio over fiber (RoF). A radio frequency signal (RF signal) or an intermediate frequency signal (IF signal) is transmitted via an optical fiber in radio over fiber. The configuration in which an intermediate frequency signal is transmitted via an optical fiber may be referred to as IFoF (intermediate frequency over fiber). IFoF is one aspect of RoF. Note that RoF or IFoF is described in, for example, documents 1-2 below.
The digital processing unit generates an intermediate signal (hereinafter “IF signal”) by, for example, up-converting a data signal. In this case, the digital processing unit converts the IF signal into an optical IF signal, and transmits the optical IF signal to the remote radio unit through an optical fiber. The remote radio unit converts the received optical IF signal into an electric signal and amplifies the electric signal. Then the remote radio unit transmits the amplified signal to a mobile station via an antenna.
In order to increase a capacity of a wireless link, a multi-antenna system that transmits a plurality of radio signals using a plurality of antennas is implemented. As an example of the multi-antenna system, a plurality of radio signals are transmitted from a plurality of transmitting antennas and received by a plurality of receiving antennas in a MIMO (Multi-Input Multi-Output) system.
In addition, a remote radio unit equipped with a plurality of transmitting antennas is proposed. In this distributed system, a plurality of signals are multiplexed and transmitted from the digital processing unit to the remote radio unit through an optical fiber. The remote radio unit respectively amplifies the signals and then outputs the amplified signals.
However, when a plurality of signals are multiplexed and transmitted, a peak-to-average power ratio (PAPR) of the multiplexed signal increases. When a peak-to-average power ratio is high, a waveform of a transmission signal may be distorted due to a saturation of a circuit element in a transmitter. For example, if a peak power of a multiplexed signal increases up to a saturation region of a laser device in a digital processing unit of the RoF system or the IFoF system, a waveform of an optical signal is distorted. In this case, communication quality may be deteriorated.
Accordingly, methods for reducing a peak-to-average power ratio have been proposed (for example, documents 3-6 below). In addition, the related technologies are described in Japanese Laid-open Patent Publication No. 2008-085379, Japanese Laid-open Patent Publication No. 2006-005390, Japanese Laid-open Patent Publication No. 2001-237800, Japanese National Publication of International Patent Application No. 2013-515424, Japanese Laid-open Patent Publication No. 2009-290493, Japanese Laid-open Patent Publication No. 2009-055558 and Japanese Laid-open Patent Publication No. 2013-153479.
The documents 1-6 are listed below.    Document 1: Charles H. Cox III et. al. “Limits on the Performance of RF-Over-FiberLinks and Their Impact on Device Design”, IEEE Translations On Microwave Theory and Techniques, vol. 54, no 2, pp. 906-920, February 2006.    Document 2: Changyo Han, Seung-Hyun Cho, Hwan Seok Chung, Sang Soo Lee and Jonghyun Lee, “Experimental Comparison of the Multi-IF Carrier Generation Methods in IF-over-Fiber System Using LTE Signals”, MWP 2014, Sapporo, Japan.    Document 3: Jose Tellado and John M. Cioffi, “Efficient Algorithms for Reducing PAR in Multicarrier Systems”, ISIT 1998. Cambridge, Mass., USA.    Document 4: H. Han and J. H. Lee, “An overview of peak-to-average power ratio reduction techniques for multicarrier transmission”, IEEE Wireless Communications, vol. 12, no. 2, pp. 56-65, April 2005.    Document 5: Mohamad Mroue, Amor Nafkha, Jacques Palicot, Benjamin Gavalda, and Nelly Dagorne, “Performance and Implementation Evaluation of TR PAPR Reduction Methods for DVB-T2”, Hindawi Publishing Corporation International Journal of Digital Multimedia Broadcasting Volume 2010, Article ID 797393, 10 pages doi:10.1155/2010/797393    Document 6: Hou-Tzu Huang et al. “W-band DD-OFDM-RoF System Employing Pilot-aided PAPR Reduction” Wireless Microwave Photonics (WMP) 2014
In the prior art, for example, a peak-to-average power ratio is reduced by “clipping” a peak power of an electric signal in a transmitter that transmits an optical signal. In this case, the optical signal is generated from the clipped electric signal and transmitted to a corresponding device (for example, a remote radio unit). However, according to this method, a waveform of a signal recovered in the corresponding device is distorted. That is, although a waveform distortion due to a saturation of a laser device may be avoided, a waveform of a signal recovered in the receiver may be distorted.